Thermal Sprayed Bioceramic Coatings: Nanostructured Hydroxyapatite (HA) and HA-Based Composites - Biological and Biomedical Coatings: Processing and Characterization - page 143

(a)

Droplet

(b)

Void

Unmelted
particle

Substrate

20 kV ×2, 000 10µm 14 4.5 Pa

FIGURE 4.2
Buildup process of a coating by accumulation of individual splats (a) and a typical cross-sectional view of plasma
sprayed zircon coating showing the layered structure. (From Gordon England, http://www.gordonengland.co
.uk/tsc.htm 2010; Li, H., unpublished data, 2004. With permission.)

significant advancement in spray technique development, and hence further improved
particle variables (i.e., temperature and velocity) have been achieved.
Among the thermal spray processes, plasma spray and HVOF are the two most widely
used processes for HA coating deposition (Figure 4.3). In plasma spraying, the torch oper-
ates with a central thoriated tungsten cathode and a water-cooled annular copper anode.

TABLE 4.1
Comparison between Different Thermal Spray Techniques Used to Produce Ha-Based Coatings

Flame HVOF APS IPS VPS RF

Gas temp. (°C)
P = 100 kPa
P = 25 kPa
P = 7 kPa

2700
-
-

3,000
-
-

14,000
-
-

14,000
-
-

-
10,000
5,000

-
8,000
-

Particle vel. (m/s)
Al 2 O 3 , 30 μm
P = 100 kPa
P = 25 kPa
P = 7 kPa

70
-
-

350
-
-

230
-
-

250
-
-

-
380
300

-
30
-

Flame length (cm)
P = 100 kPa
P = 25 kPa
P = 7 kPa

<10
-
-

<20
-
-

<7
-
-

<10
-
-

-
<15
<50

-
<15
-

Part. Injection mode Axial Axial Perpendicular to the plasma jet and

downstream of the arc

Axial